In the past, many designs have addressed the problem of casting a molten metal into a moving mold, continuously, with an operable and reliable machine. Towards this end, about 20 years ago, Lauener disclosed a continuous casting machine in U.S. Pat. No. 3,570,586, which used multiple mold block assemblies to form a continuous mold cavity from which molten metal emerged as a slab casting. Because the machine uses mold blocks it is referred to as a "block caster", and because it continuously casts a slab (or "strip") it is also referred to as a "slab caster". The uphill struggle by many persons skilled in the art, to establish the Lauener machine as a viable contender against competing commercial "belt casters", has over the years, spurred much effort towards refining concepts incorporated in machines able to produce a quality slab casting, reliably and economically.
This invention is specifically directed to separate supporting and fastening means which are together referred to as "mounting means", employed for attaching a mold block to a carriage block in any continuous block caster. An internally anchored mold block, and a support pin which anchors the mold block through its "non-casting face", have been designed to minimize the effects of thermal deformations to which mold blocks are subjected in the block caster. By "non-casting face" we refer to the horizontal face of a mold block which does not contact molten metal; the horizontal face which does, is referred to as a "casting face"). The interaction of the elements of the mounting means and the internally anchored mold block is unobviously effective to cope with such effects, particularly the vertical expansion, which is minimized to such an extent that no compensation for the expansion which does occur, is necessary.
As will presently be evident, the goal of this invention is to provide a block caster able to cast a slab of uniform thickness with unstriated, substantially smoothly planar upper and lower surfaces formed between upper and lower reference planes defined by the casting faces of opposed mold blocks. To cast such a slab, the casting faces of the opposed mold blocks are to be maintained in their corresponding reference planes while they travel through the mold cavity of the block caster.
In a block caster, a pair of synchronously driven endless trains of mold block assemblies ("casting trains") travelling in paths which resemble loops, define a substantially linear horizontal mold cavity having open ends, when the opposed "casting faces" of mold blocks in the mold block assemblies come together in spaced-apart relationship, facing each other, along the linear portions of the loops. Each loop has its linear portion connected by upper and lower arcuate portions or "bends" which complete the loops, and the mold block assemblies are endlessly interconnected and oppositely disposed relative to one another in the adjacent linear portions of the loops. The mold cavity is preferably defined in conjunction with "side dams" which together confine the molten metal in the moving mold cavity.
A casting nozzle is inserted near one open end, referred to as the "molten end", to supply the mold cavity with molten metal from a tundish. The molten metal cools as it progresses with the mold block assemblies until the slab emerges from the other open end, referred to as the "solid end". The longitudinal direction in which the molten metal is cast is referred to as the "casting direction" or the x-axis. The lateral direction, orthogonal to that in which the metal is cast, is referred to as the "transverse direction" or y-axis; and the vertically spaced-apart distance of the mold block faces which define the thickness of the cast slab is said to be in the vertical direction, or z-axis.
As one might expect, to cast a slab with uniformly rectangular cross-section, it is essential that, in the mold cavity, the faces of the mold blocks defining the upper surface of the slab be in the same plane, and that the faces of those mold blocks defining the lower surface of the slab be in the same plane. When any portion of the face of a mold block is displaced from its original planar configuration, the slab cast will not have planar upper and lower surfaces. Depending upon the type of displacement, the surfaces will be arcuate, rippled or striated. When the mold blocks are not contiguous, or, if their vertical sides are not planar when they are touching, the surface of a cast slab will be striated, referred to as "ribbed"; if the edges of the blocks are thermally distorted so as to result in "bumps" (explained herebelow), the surface of a cast slab will also be striated, said to be "grooved". Whether ribbed or grooved, if present, such striations are clearly visible in the cast slab, for example as seen in the color photograph in the article titled "Small Aluminum Companies Speed Up, Spread Out" by George McManus, Iron Age, Feb '91.
Portions of the mold block faces are unavoidably displaced because they get heated to a high temperature when they contact molten metal at the molten end, then cool progressively as they transfer heat to the mold block as it reaches the solid end. The mold blocks are necessarily distorted due to the temperature gradients which are three-dimensionally distributed through the mold block. As the mold blocks cool and the molten metal solidifies, their original dimensions begin to be restored, returning to normal when sufficiently cooled, if the edges of blocks are undamaged.
Under actual operating conditions, the distortions of the mold blocks are such that they exert enormous pressure against contiguous mold blocks forcing the metal of the blocks out of their planar conformance in edge-abutting protuberances, referred to as "bumps". These bumps interfere with the smoothly planar definition of the surfaces of the upper and lower series of mold block faces. As a result, the upper and lower surfaces of the slab are neither planar nor smooth (that is, have poor "surface accuracy"). Such a slab is unacceptable in commerce because its surface contains cracks, or, provides locations from which cracks can propagate when the slab is rolled. A slab with poor surface accuracy is evidence of the "casting problem"--the less accurate the surface, the greater the problem.
The design and construction of a block caster derives from a fundamental decision whether to restrain the forces of distortion by equal and opposite restraints, or, to control and limit the distortions without substantially restraining them, and to cope with the controlled distortion.
We decided that the first step towards solving the well-recognized problem of poor surface accuracy was to provide a machine designed to allow the mold blocks to undergo their cyclical thermal changes without substantially restraining them, yet without interfering with the continuously planar configuration of the opposed surfaces of the mold cavity.
To this end, in our copending patent application Ser. No. 07/674,664 we provided "microslits" in the face of each mold block. We minimized the "mechanical noise", for example, the vibrations transmitted to the mold blocks during operation, by providing elastic hinges which maintained the faces of mold blocks in each carriage track, in or near the open ends of the mold cavity, in essentially contiguous relationship even in the "bends" of each continuous carriage track. By "essentially contiguous" we mean that casting faces of mold blocks are horizontally spaced apart ("gapped") less than 0.020" (inch) preferably less than 0.005"; and, vertically spaced apart ("set") less than 0.020", preferably less than 0.005", so as to form a smoothly continuous arcuate surface in the bends. The elastic hinges eliminated the mechanical excitation (familiarly referred to as "banging") caused by impact in the "bends" of a "loop", of adjacent mold blocks having radially divergent corners, and circumferentially spaced-apart faces.
Because a mold block supported from a carriage block, must be replaceable in any block caster irrespective of the details of the particular design and construction of the block caster, we have provided for replacement of a mold block, together with the support pin assemblies which support the mold block, quickly and easily. We have invented a method for substituting a `fresh` (new or remanufactured) mold block assembly for one which requires replacement even in a block caster in which mold block assemblies are interlinked as if in a chain (hence referred to as "chain-wise linking"). Such chain-wise interlinking is unlike the linking in the '586 patent which teaches separate mold blocks connected so that removing one mold block allows the remaining mold blocks to separate from one another in the train. In chain-wise interlinking, removing a mold block to separate the ends of the chain, allows one to hold all the mold blocks supported by only the first mold block at one end of the chain. Because of the inherent difficulty of replacing a mold block in this chain-wise configuration, which is the linking in the most preferred embodiment of the invention, it is particularly surprising that a substitution may be made in a manner as effective as we have devised.
Despite the features we have disclosed for a block caster in our '664 application, which features redound to superior operation of that block caster, we found that the conventional mounting means, such as are disclosed in the aforesaid Lauener '586 patent, fail to provide a sufficiently high quality cast slab, reliably and routinely. By "sufficiently high quality" we refer to a slab which is acceptable to a customer who purchases slabs for his further use, for example, to be rolled into "can stock" for beverage cans.
Briefly, the mounting means used in the '586 patent secures a mold block to a guide member. The mounting means relies upon a bolt 15 one end of which is threadedly secured in the geometric center of a rectangular mold block 10; the other end of the bolt 15 is passed through a bore in a guide member 11 and tightened against it with a nut 16 and an interposed spring washer (cup spring) 17. Four supports (bolts) 12, two on either side of the bolt 15, are provided with spherical rounded ends which rest on backings 13 of hardened steel. The backings 13 are in the form of plane discs having a raised rim and the rounded ends of the bolts are in contact with the planar discs so that when the mold block 10 expands or contracts the bolts 12 assume a sloping position. Thus when the mold block does move laterally, it causes rotation of one spherical rounded end of each bolt 12 (in a backing 13 which is sunk in the mold block 10) and a simultaneous, corresponding rotation of the other spherical rounded end of the bolt in another backing 13 sunk in the guide member 11. This causes a displacement along the diameter of the sphere (of which the spherical rounded ends of the bolts are opposite portions), and there is no change in the distance between the mold blocks and the carriage block (or `guide member`, see col 3, lines 61-75).
In the '586 patent, Lauener expected to maintain the distance between the non-casting face of the block and the carriage block constant with the aforesaid bolts with spherical rounded ends. But these bolts are unable to accommodate anything but the smallest vertical expansion even with the assistance of the spring washer (cup spring 17). A large, if not major portion of the vertical expansion of the mold block occurs after the cast slab solidifies. Under such conditions, the casting face of the block is positioned against the solidified slab and cannot move, forcing movement of the non-casting face of the block away from the slab. This exerts great pressure on the carriage block and associated structure of the block caster, which pressure forces the casting face of a mold block in the processing zone out of the reference plane for the faces of mold blocks of each train in that zone. Such pressure also accelerates distortion of the casting face of the mold block. The bolts with spherical rounded ends and the spring washer of the '586 structure are ineffective to cope with the relatively large vertical expansion which results after solidification of the slab.
The foregoing will be better understood upon envisioning the sequence of events in the processing zone. At the start, when molten metal contacts the casting faces of specific opposed mold blocks at the mouth of the mold cavity, the thermal gradient between the casting face and non-casting face of each of those molds block is greatest. The initial expansion of each mold block due to this gradient, forces each casting face out of its reference plane for the casting face of that mold, immediately. Since the melt offers little resistance, the opposed casting faces of those mold blocks move towards each other, squeezing the cooling melt into a thinner (measured along the z-axis) slab than when the faces of the mold blocks first commenced to form the mold cavity.
As the melt cools while travelling through the mid-portion of the mold cavity, those specific mold blocks continue to expand vertically and laterally. Such vertical expansion progressively further forces the casting faces of those opposed mold blocks towards each other, and forces each casting face out of its respective reference plane, so long as the central portion of the cooling slab is relatively soft and compressible, though the slab, near its surface, is solid.
As the cooling slab continues to travel through the mold cavity, the thermal gradient between the casting and non-casting faces of those mold blocks is decreased relatively rapidly because of the typically high thermal conductivity of any metal preferred for use as a mold block. At this later stage in the travel of the slab through the mold cavity, the slab is solidified and essentially incompressible so that further vertical expansion of each of those mold blocks results in force being exerted against the carriage block assembly, thus "loading" it in proportion to the magnitude of the force exerted. It is this force exerted against the solidified slab which causes displacement of the backings 13 and rotation of the spherical rounded ends of the bolts 12 in the block caster of the '586 patent. Near the far end of the mold cavity where the slab is solidified, the displacement of opposed casting faces towards each other is essentially zero because the slab is incompressible, and, the displacement away from each other is relatively small compared to the displacement of those casting faces in their travel through the initial and mid-portions of the mold cavity, because of the limitations imposed by the bolts 12 in their backing plates.
The mold block 10 in the '586 patent is maintained in the central position relative to the guide member 11 by means of three guide pieces 21, 22 and 23 screwed on to the guide member and slide in grooves 18, 19 and 20. The guide piece 23 prevents lateral displacement (along the y-axis), and guide pieces 21 and 22 prevent displacement in the casting direction (x-axis).
As will be evident from the placement of the bolts 15 which externally anchor the surface of the mold block 10 to the guide member 11, during operation of the block caster described in the '586 patent, the displacements due to the vertical expansions referred to hereinabove will correspond to the vertical expansion of the entire block. In contrast, in the detailed description of a preferred embodiment of an internally anchored mold block herebelow, the displacement due to such vertical expansion is limited to the thickness of only that portion of the mold block near its face, which portion is defined by the distance (referred to as the "heat-sensitive distance") between the casting face and the lateral centerline of an anchor for a support pin of a support pin assembly. When this heat-sensitive distance is less than one-half the height or thickness (measured along the z-axis) of the mold block, preferably one-third, the vertical expansion attributable to such distance is less than one-half that attributable to the entire thickness of the mold block.
It will now be evident that Lauener (in '586) failed to accommodate the vertical expansion of the mold block and its supporting means by the spring washer (cup spring 17). The spring washer and spherical rounded ends of the bolts 15 could only accommodate a portion of the lateral expansion of the block by angulation of the bolts 15. As did others in the prior art, Lauener failed to make provision to cope with relatively large changes in positions of the casting faces of opposed mold blocks as they are forced towards each other while the slab is compressible, or as they are forced away from each other after the slab is solidified.
The positions of the casting faces are not fixed prior to solidification of the slab, but "float" up and down due to thermal changes. For casting aluminum, this vertical expansion (in the direction of the z-axis) is about 0.32 mm for a mold block of practical thickness (say 10 cm, measured along the z-axis), and is too great to be accommodated by any prior art vertical compensating means.
We have found that it is most desirable to maintain the relative positions of opposed casting faces of mold blocks substantially coplanar in their respective reference planes throughout the casting zone, and to do so, the vertical displacement of these faces relative to each other must be minimized. We have been able to do this by supporting each mold block with an insulated support pin anchored within the mold block, relatively close to the casting face of the mold block, so that such vertical distortion as is transmitted to the support pin is too little to cause undesirable striations in the surface of the cast slab. Thus the casting faces of contiguous mold blocks of each loop, maintain a substantially coplanar configuration in the casting zone, which coplanar configuration in turn, results in substantially smoothly planar upper and lower surfaces of a cast slab which is commercially highly desirable for further processing.